This invention relates to apparatus for distributing power from a single a.c. source to a plurality of consuming units, more particularly electrical appliances, or energy consuming elements within a single appliance, which appliances or elements are operated so as to have an average consumption of electrical energy which is much lower than their maximum consumption.
In a conventional electrical installation in which a number of appliances are fed from a common supply, and in which all the appliances may be turned on at the same time, it is usually necessary for the supply wiring to be capable of sustaining, and the supply to be capable of supplying, the total maximum power demand of all the appliances operating simultaneously. Moreover, in large electrical installations the tariffs charged by electricity supply authorities are often dependent, sometimes very markedly so, upon the peak power demand of the installation. The desirability of reducing peak power demand whenever possible is thus manifest.
It is a feature of many types of electrical equipment, in particular much equipment incorporating electrical heating elements, that the maximum power demand is much greater than the normal average power demand. Such equipment is usually thermostatically controlled and under normal running conditions the "off" periods of the thermostat are long compared with the "on" periods. Nevertheless, the wiring of an installation containing several units of such equipment must provide for the possibility of all the thermostats being on simultaneously. Similar considerations apply to an appliance containing several independently controlled heating elements, such as an electric cooker.
It has been proposed to provide installations in which the maximum loading on the supply is controlled by arranging that an increasing number of portions of the load are disconnected by a "stabilizer" as their combined demand increases so as to restrict total consumption to a predetermined level, the order in which such portions are disconnected being according to a predetermined scheme of priorities. It has also been proposed that load portions of equal priority in such an arrangement should be separately connected to the stabilizer through a commutating arrangement so that their positions in the priority scale are periodically exchanged and in the long term, power is distributed to such equal priority loads according to the mean priority level of the connections between the commutating arrangement and the stabilizer.
A disadvantage of such systems of this nature that have been proposed is that the various load portions are wholly disconnected from the supply when load shedding takes place, and also the switching operations involved, although maintaining demand below a maximum level, will give rise to transient fluctuations in loading which not only may give rise to radio interference but also may cause undesirable fluctuations in the supply voltage, particularly if the loads being switched are large.
Power control systems for alternating current supplies are well known which make use of thyristors and other related semiconductor devices such as triacs. Such systems are of two main types. The first, phase control, type relies on varying the point in each, or each alternate, half cycle of the supply at which the control device is switched on. This type of control presents severe radio interference and power factor problems and is not favored for installations having a substantial power consumption. The alternative "burst-fire" system, in which control is exercised by controlling the frequency and or length of periods or "bursts" for which the control device is switched on for an integral number of half cycles, overcomes the problems of the phase control method but does apply a fluctuating load to the supply. Moreover, if the duration of the burst is relatively short the frequency of this fluctuation is particularly objectionable.